1. Field of the Invention
The present invention relates to a resonator, and more particularly to a resonator of 1/4 wavelength which has a pattern electrode and used in a chip type filter.
2. Description of the Prior Art
FIG. 17 is a perspective view showing an example of a conventional resonator. The resonator includes a dielectric substrate 1. An earth electrode 2 is formed almost entirely on one main surface of the dielectric substrate 1. A straight pattern electrode 3 is formed on the other main surface of the dielectric substrate 1 so as to be opposite to the earth electrode 2. One end of the pattern electrode 3 is connected to the earth electrode 2 via an end face of the dielectric substrate 1. A micro strip line is formed of the dielectric substrate 1, the earth electrode 2 and the pattern electrode 3.
FIG. 18 is a perspective view showing an example of a conventional chip type filter. The chip type filter includes a dielectric substrate 1. An earth electrode 2 is formed almost entirely on one main surface of the dielectric substrate 1. Two straight pattern electrodes 3a and 3b are formed on the other main surface of the dielectric substrate 1 so as to be opposite to the earth electrode 2. One end of each of the pattern electrodes 3a and 3b is connected to the earth electrode 2 via an end face of the dielectric substrate 1. The pattern electrodes 3a and 3b are formed so as to be parallel with each other and electromagnetically connected with each other. Drawing electrodes 4a and 4b are formed from the pattern electrodes 3a and 3b toward both ends of the dielectric substrate 1, respectively. Each of the drawing electrodes 4a and 4b is formed with a fixed interval spaced from one end of each of the pattern electrodes 3a and 3b connected to the earth electrode 2. In the chip type filter, a filter is constituted of an electromagnetic connection between two micro strip lines.
In each of the resonator shown in FIG. 17 and the chip type filter shown in FIG. 18, it is desired to increase Q without enlarging in a plan configuration.
Each of the length L of the pattern electrode of the resonator shown in FIG. 17 and the length L of the pattern electrode of the chip type filter shown in FIG. 18 is shown by the following equation (1). EQU L=.lambda./(4 .epsilon..sub.re.sup.1/2) (1)
where .lambda. is a wavelength, and .epsilon..sub.re is the effective dielectric constant of the dielectric substrate. As shown in the equation (1), in order to miniaturize the resonator and the chip type filter, it is conceivable to use a dielectric substrate having a high dielectric constant. However, in consideration of its temperature characteristic, it is impossible that the dielectric substrate has a very large dielectric constant. The upper limit of the dielectric constant .epsilon..sub.r of the dielectric substrate is approximately 100. Supposing that the dielectric constant can be replaced with the effective dielectric .epsilon..sub.re, the length L of the pattern electrode in the resonator and the chip type filter of 1 GHz is calculated as long as 7.5 mm from the equation (1). Thus, it is difficult to miniaturize a resonator and a chip type filter of a high frequency band, for example, 1 GHz-3 GHz. In a chip type filter having a plurality of pattern electrodes electromagnetically connected with each other, the electromagnetic connection therebetween becomes greater as the lengths of the pattern electrodes increase. Thus, it is necessary to enlarge the interval between the pattern electrodes, which causes the chip type filter to be very large.
Then, a resonator and a chip type filter can be miniaturized are devised.
FIG. 19 is an exploded perspective view showing an example of a resonator which can be miniaturized and is the background of the present invention. In the resonator shown in FIG. 19, compared with the resonator shown in FIG. 17, the pattern electrode 3 is spirally formed. The pattern electrode 3 is formed so that the width thereof becomes stepwise smaller from the outermost end connected to the earth electrode 2 toward the innermost end opened.
FIG. 20 is an exploded perspective view showing an example of a chip type filter which can be miniaturized and is the background of the present invention. In the chip type filter shown in FIG. 20, compared with the chip type filter shown in FIG. 18, two pattern electrodes 3a and 3b are spirally formed so that the widths thereof become stepwise smaller from the outermost ends connected to the earth electrode 2 toward the innermost ends opened.
The resonator shown in FIG. 19 and the chip type filter shown in FIG. 20 can be miniaturized, because the pattern electrodes are spirally formed. Further, the resonator and the chip type filter have a high Q, respectively. Because, in the resonator and the chip type filter, the electric current distribution becomes smaller from one end of the pattern electrode on the earth electrode side toward the other end thereof opened, whereas the width of the pattern electrode becomes smaller from one end thereof toward the other end thereof. Thus, it is possible to obtain a resistance corresponding to the electric current distribution.
However, in the resonator shown in FIG. 19 and the chip type filter shown in FIG. 20, in accordance with miniaturizing, the width of the pattern electrode is reduced, and Q can not be further increased. Because, in the resonator and the chip type filter of high frequency band, electric current is concentrated on the surface of conductor such as the pattern electrode due to skin effect, and hence, the resistance component of the pattern electrode becomes high as a result of the reduction in the width of the pattern electrode.